In display devices such as an organic electroluminescence display device and a liquid crystal display device, methods like sputtering, evaporation, or CVD are generally applied to form an organic or inorganic functional film. Recently, various functional films can be formed by coating. Particularly, a hole injection layer (HIL) in the organic electroluminescence display device and an alignment film (e.g., a polyimide alignment film) in the liquid crystal display device are usually coated via a die slit coater (DSC), an inkjet printer (IJP), or the like.
Since a thickness of the coated functional film greatly influences the function of the display device, a good solvent is usually adopted in which a functional material (a precursor for the functional film) dissolves easily, so as to uniformly form a functional film on a substrate. Besides, to improve wetting property of the solvent on the substrate surface and impart the dried functional film a uniform profile, a poor solvent is further applied so that the coated functional film has a uniform thickness.
A die slit coater DSC shown in FIG. 1, an inkjet printer IJP with a piezoelectric element shown in FIG. 2, and an electric spray coater ESC shown in FIG. 3 are usually applied to form a functional film by coating.
FIGS. 1A, 1B, and 1C are schematic views for illustrating how to coat a functional film on a substrate by a die slit coater DSC. A substrate 104 is fixed to a submount 102. A coating component 106 advances in a direction indicated by an arrow M, and coats an ink comprising a functional material onto the substrate 104. After being dried, the functional ink 108 forms a functional film 108 on a surface of the substrate 104. It is noted that in this context, the functional ink and the functional film made from it share the same or similar reference numerals for simplicity.
FIGS. 2A and 2B are schematic views for illustrating how to coat a functional film on a substrate by an inkjet printer IJP. A substrate 204 is fixed to a submount 202. A coating component 206 advances in a direction indicated by an arrow M, and prints an ink comprising a functional material onto the substrate 204. The ink spreads to form a film, and once being dried, forms a functional film 208 on a surface of the substrate 204.
FIGS. 3A and 3B are schematic views for illustrating how to coat a functional film on a substrate by electric spray coater ESC. A substrate 304 is fixed to a submount 302. A coating component 306 advances in a direction indicated by an arrow M, and coats charged particles of a functional material on the substrate 304. The charged particles form a film in a region delimited by a boundary 307, and once being dried, form a functional film 308 on a surface of the substrate 304.
Unfortunately, an expensive functional ink is required in these coating apparatuses. To reduce the consumption of ink, it is desired to only coat in the required portions.
By coating with an inkjet printer IJP in an on-demand manner, it is not only possible to specify a region to be coated, but also to coat only in the region to be coated. By using a split-type die slit coater DSC in which the slit width is divided shown in FIG. 1C, coating can at least partially be performed in the on-demand manner. An electric spray coater ESC which uses a mask for delimiting a region to be coated will be available in the near future.
On one hand, in view of the function of the display device, a peripheral bezel region outside the active area is becoming smaller and smaller. On the other hand, narrow bezel display devices become increasingly popular in recent years. FIG. 4A is a top view for illustrating a substrate 404 on which a functional film 408 has been coated, FIG. 4B is an enlarged view for illustrating a corner portion 401 in FIG. 4A, and FIG. 4C is an enlarged view for illustrating a corner portion 402 in FIG. 4B. As shown in FIGS. 4A-4C, a functional ink 408 coated on such narrow bezel display devices starts to spread from a portion with relatively good wetting property on the substrate 404, and spreads in a flat manner to form an irregular spreading front. Since the coated functional ink does not spread uniformly on the substrate 404, so that the finally formed functional film has a non-uniform thickness, leading to defects like mura in light emitting in the finished display device. Besides, apart from the active area 410, the functional ink 408 may further cover contact holes 430 in the peripheral region. It was proposed to remove the functional film formed by the functional ink 408 which spreads to undesired regions by plasma or laser. However, in this case, sublimation during removal by plasma or laser may affect functional components in the display device, making it not the best way to remove the functional film.
There is a need for an improved display device and a method for fabricating the same in the art.